1. Field of the Invention
The present invention relates to a temperature control method in hot pressing, in which the temperature of a heating plate is controlled by use of a heating medium such as vapor, hot oil or water.
2. Description of the Related Art
A hot pressing apparatus in which a temperature control method according to the present invention is carried out is generally indicated at 1 in FIG. 1. The structure of the apparatus shown in FIG. 1 is identical to a hot pressing apparatus for a conventional temperature control method. In the hot pressing apparatus 1, a plurality of heating plates 4 that are equally spaced are arranged between a movable platen 3 and a stationary platen 2 when the movable platen 3 is most distant from the stationary platen 2. Workpieces are placed on the heating plates 4 and, then, the movable platen 3 is moved close to and pressed against the stationary platen 2 by a pressing device (not shown) to form the workpiece. At this time, a heating medium whose pressure or flow rate is controlled by a control valve 23, is supplied from a heating medium source 24 through an inlet manifold 5 to heat or cool each heating plate 4 and, thereafter, is collected by an outlet manifold 8 and is returned to the heating medium source 24. In a heating-plate-temperature control of the conventional hot pressing apparatus, temperature or pressure sensors are provided at the inlet manifold 5, the outlet manifold 8 or the heating plates 4. The control valve 23 is controlled by a feedback-control, only based on detection values of a sensor 6, a sensor 9 or sensors 11 to 14.
In the conventional control method, the control can be stably carried out in a temperature-setting pattern (maintaining step 18) in which the temperature is constant in the lapse of time, because there is no deviation between a preset value and a measured value. However, there is a deviation between a preset value and a measured value in a temperature-setting pattern, such as a temperature increasing step 17 or a cooling step 19, in which the temperature varies in accordance with the lapse of time. Especially, if variations in temperature, i.e., the temperature gradient in the temperature-setting pattern is large, the measured value varies, with a delay, with respect to the preset value and an overshoot or undershoot of the measured value occurs at the transfer point between the temperature increasing step 17 or the cooling step 19 and the maintaining step 18.
The reasons that such a phenomenon occurs are as follows. Namely, if the heating medium is hot oil or water, the heat exchange is enhanced as the flow rate of the heating medium is increased and, thus the control accuracy of temperature is improved. However, it is impractical to remarkably increase the flow rate, in terms of cost performance because a large and expensive pump is required to increase the flow rate of the heating medium. Large diameter pipes and the inlet manifold 5, etc., are provided in a heating medium passage between the control valve 23 and the heating plates 4 and, thus the control is retarded by a time corresponding to passing of the heating medium remaining in the pipes and the inlet manifold 5 through the heating plates 4.
The above problem is more serious when the sensor is located at the outlet manifold 8. To solve this problem, the sensor can be provided at the inlet manifold 5, instead of at the outlet manifold. However, in this solution, another problem arises. Namely, the control is carried out so that the temperature of the heating medium before entering the heating plates 4 is equal to a preset value. Therefore, the temperature of the heating plates varies in accordance with a change in heat capacity due to a difference in the size or material of workpieces. In addition, even if the size or material of the workpieces are the same, when workpieces are not placed on all the heating plates, the heat capacity decreases in accordance with the amount of decrease of the workpiece and, then the temperature of the heating plate tends to increase.
In recent years, a rapid cooling speed that had not been previously required has been needed in forming of the workpieces. If such rapid cooling is controlled by feedback based on the temperature of the heating medium only, the control is carried out indirectly, thus leading to an inevitable delay in control. To avoid this problem, the cooling can be directly controlled by feedback of the temperature of the heating plate. However, in such a heating medium control system, it is very difficult to stably control all the steps.
According to the present invention, any two temperatures from among a temperature of the heating medium in an inlet manifold that distributes and supplies the heating medium to each heating plate, a temperature of the heating medium in an outlet manifold that collects the heating medium discharged from the heating plates and a temperature of the heating plates, are detected; the two temperatures are selected in accordance with a temperature-setting pattern; and feedback-control is carried out so that the selected temperature matches the temperature-setting pattern. With this structure, a control loop having an optimum temperature-detecting portion suitable for steps to be controlled can be constructed, and a precise temperature control can be carried out in each step.